1. Field of the Invention
The invention relates to an engine control system for a direct injection-spark ignition type of engine and, in particular, for a direct injection-spark ignition type of engine equipped with an exhaust gas recirculation system which controls fuel injection timing in an engine operating zone in which the exhaust gas recirculation system is actuated to admit exhaust gas into an intake air stream.
2. Description of the Related Art
Engine control system of this type incorporate in an exhaust line a NOx adsorbing type of lean NOx conversion catalyst which, on one hand, adsorbs NOx in the exhaust gas while the air-fuel mixture is leaner than a stoichiometric mixture and, on the other hand, desorbs or releases the NOx into exhaust gas for catalyzing reduction of the NOx while the air-fuel mixture is richer than a stoichiometric mixture. As is known from, for example, International Patent Application W093/07363, such an engine control system controls the engine to operate with an enriched mixture under accelerating conditions or full loading operating conditions and with a lean mixture under the remaining operating conditions, so as to improve fuel consumption.
An engine control system for a direct injection-spark ignition type of engine known from, for example, Japanese Unexamined Patent Publication 7-119507 controls the engine to cause stratified charge combustion in a lower engine loading zone and homogeneous charge combustion in a high engine loading zone. While the engine operates with lower speeds in the high loading zone, a given amount of fuel is delivered in two steps through early and late split injection in a intake stroke, so as to diffuse a first half of fuel sprayed through the early split injection in the combustion chamber before the end of a intake stroke and the second half of fuel in the combustion chamber with its volume increased, This prevents a generation of rich or dense mixture over the top of a piston in a subsequent compression stroke that generally occurs when a given amount of fuel is sprayed all at once through non-split injection, which is desirable to prevent generation of smoke.
Another engine control system for a direct injection-spark ignition engine cooperates with a fuel injector which is direct to face the top of a piston and energized to spray a small amount of fuel preparatorily at the beginning of a intake stroke when the engine causes knocking. The fuel partly sticks to the top wall of the piston on a side of an intake port and bounces off the piston wall toward the intake port to cool the piston head and the combustion chamber on the intake port side with the heat of vaporization of the fuel. Such an engine control system is known from, for example, Japanese Unexamined Patent Publication 7-217478.
While a lean NOx conversion catalyst disposed in an exhaust line as described in the International Patent Application W093/07363 lowers an emission level of NOx in lean exhaust gas, it shows a catalytic conversion efficiency insufficient as compared with the NOx conversion efficiency of a three-way catalyst at a stoichiometric air-fuel ratio. In particular, a NOx adsorption type of lean NOx conversion catalyst lowers its NOx conversion efficiency due to an increased amount of NOx adsorption resulting from a long time continuous operation of the engine with a lean fuel charge. Accordingly, it is necessary to enrich the fuel charge before a specified amount of NOx adsorption is exceeded. In other words, the engine operation with a lean fuel charge is restrained by the necessity of refreshing the lean NOx conversion catalyst, which is undesirable for fuel consumption.
In order to control the formation of NOx, it has been known to lower the maximum temperature of exhaust gas by recirculating exhaust gas partly into an intake air stream. However, if admitting a large amount of exhaust gas into an intake air stream, drivability is possibly lowered due to an increased combustion change. If controlling the amount of exhaust gas recirculation so as not to exceed a limit for preventing or significantly reducing aggravation of drivability, it is hard to satisfy the demand for lowering the formation of NOx and improving fuel consumption.